Electrosurgery generally refers to surgery using electricity to achieve a certain effect at one or more target tissue sites in a patient. Typically, electrosurgery involves using high-frequency electrical energy, such as radio frequency (RF) energy, to cut or dessicate tissue, coagulate blood to stop bleeding from small blood vessels, cause tissue necrosis, and the like. For example, high-frequency electrical energy may be used with an electrosurgical scalpel to start or deepen an incision and/or to coagulate small blood vessels that are cut during incision. In another example, high-frequency energy may be delivered to diseased regions in target tissue, such as cancerous metastases in a liver, to cause necrosis of the diseased regions.
Equipment for performing electrosurgery generally includes a high-frequency electrical energy source, an active electrode, a dispersive electrode sometimes called a “return electrode”), and wiring for connecting the energy source to the active and dispersive electrodes. The high-frequency energy source generally supplies a high-frequency current to the active electrode, via the wiring, and the active electrode is generally used to apply the current at a target tissue site on a patient for performing an electrosurgical task. Typically, the active electrode is relatively small in surface area, relative to the dispersive electrode, so that relatively high current density is delivered by the active electrode. The high-frequency current travels from the active electrode, through the patient, to the dispersive electrode. Both the active and dispersive electrodes are attached to the energy source via one or more cables. Thus, a typical electrosurgery system may include an electrical circuit including an energy source, cable to active electrode, patient, dispersive electrode, and cable to energy source.
There are two general types of electrosurgical systems currently in use, namely bipolar and monopolar systems. In bipolar electrosurgery systems, both electrodes may be similar in surface area and are positioned in close proximity to one another, e.g., together on the same, handheld electrosurgical device. For example, some bipolar devices are configured as forceps (similar to tweezers) with the distal end of one prong of the forceps being a first active electrode and the distal end of the other prong being a second active electrode. Bipolar surgical systems are widely used for coagulating tissue, especially in procedures where it is important to prevent electric current from flowing through tissues adjacent to the target tissues.
Monopolar systems, however, are much more commonly used than bipolar systems. In monopolar electrosurgery, the active and dispersive electrodes are typically separated by a significantly greater distance than the electrodes in bipolar electrosurgery systems. Both electrodes are connected to a high-frequency energy generator. FIG. 1 schematically illustrates a monopolar electrosurgery system 100 that may include a high-frequency electrical energy source 102, an active electrode cable 104, an active electrode 106, a patient 112, a dispersive electrode 108, and a dispersive electrode cable 110.
In any type of electrosurgery, if a return electrode or other return path were not provided so that electrical current delivered to a patient by the active electrode could not readily return to the energy source to complete the circuit loop, the system would not work. Moreover, even if a return path is provided, current from an electrosurgical device may harm a patient if the return path is flawed in some way. When a return path malfunctions in monopolar electrosurgery, an unwanted patient burn may result. Patient burns at locations other than the target tissue site typically occur when current returning from the active electrode to the energy source becomes too concentrated (dense) at the location where the current leaves the patient. As described briefly above, a monopolar active electrode is typically relatively small, often only one or two millimeters (1-2 mm) or less in diameter at its operative, distal end. When current from an energy source is delivered to such a small device, a high current density is produced that can be used for ablation, coagulation, necrosis, and the like at the target site.
Dispersive electrodes, in contrast, are designed to have significantly larger surface areas. For example, a commonly used dispersive electrode is the Valleylab™ PolyHesive™ Patient Return Electrode, available from Valleylab (Boulder, Colo.), a division of Tyco Healthcare LP. A Valleylab™ PolyHesive™ Patient Return Electrode generally includes a thin, flexible, adhesive pad, measuring approximately seven inches by four inches (7″×4″), with an attached electrical cable for connecting to an RF energy source. The large surface area of such a patient return electrode, when compared to the surface area of the active electrode, causes current flowing out of the patient through the return electrode to have a relatively low current density. The low current density is intended to prevent excessive heating or burning of the patient's skin at the return electrode/skin interface.
Generally, dispersive electrodes for monopolar electrosurgery (also referred to as “patient return electrodes”, “return pads” or simply “pads”) work sufficiently well to disperse current and return it from a patient to a generator. Occasionally, however, a dispersive pad may malfunction, causing high density current at the dispersive pad/patient interface and possibly a patient burn. For example, the wiring of a dispersive pad may malfunction, a dispersive pad may be improperly placed so that it contacts the patient with a smaller surface area than intended, a dispersive pad may partially fall off the patient during a procedure, and the like.
The risk of unwanted patient burns in monopolar electrosurgery has become increasingly important as monopolar surgery devices have become increasingly more powerful. With the advent of active electrodes with larger surface areas, such as the LeVeen™ family of electrodes (distributed by Boston Scientific Medi-Tech, San Jose, Calif.), typical impedance loads for electrosurgery generators have decreased. A decreased load requires a generator to provide increased current in order to deliver a given amount of energy (watts). For example, the RF-2000™ and RF-3000™ Radiofrequency Generators (also distributed by Boston Scientific Medi-Tech) provide up to one hundred Watts (100 W) and two hundred Watts (200 W) of radio frequency energy, respectively.
High-current monopolar surgery devices may provide advantages in many electrosurgical procedures. A monopolar electrode, such as the LeVeen™ Needle Electrode or CoAccess™ Electrode (also available from Boston Scientific Medi-Tech), used with an RF-2000™ or RF-3000™ may be particularly advantageous for ablating and/or necrosing tissue in a cancerous tumor in a solid organ such as cancerous metastases in the liver.
Some currently available monopolar electrosurgery systems may include one or more safeguards against unwanted patient burns. For example, some systems may include multiple dispersive electrodes to disperse current from a patient. These systems represent an improvement over one-pad systems, but a risk still exists that current flowing through two or more dispersive electrodes on a patient may be unbalanced. Even if multiple pads are used to disperse current, a patient burn may still occur if current flowing to one pad is sufficiently high to cause such a burn.
Therefore, improved monopolar electrosurgical methods and apparatus would be useful.